The present invention provides a way to dehydrate and enrich aqueous alcohol mixtures such as ethanol, in which the combustion energy of the ethanol product exceeds the energy needed to carry out the dehydration by a factor of 10. Drying of aqueous ethanol by materials such as cellulose, cornstarch, shelled corn, or corn (cellulosic) residue, results in a product that is up to 99.8 percent water-free.
Alcohols are made from either grains or biomass by first converting these materials to fermentable sugars. The sugars are then fermented, typically with yeast, to give a broth containing 6 to 12 percent ethanol along with small amounts of aldehydes, ketones, amyl alcohols (fusel oils), and methanol. The final step, distillation to water-free alcohol, consumes 50 to 80 percent of the energy used in a typical fermentation ethanol manufacturing process. The energy intensity of traditional distillation techniques is frequently cited in criticizing the potential of biomass-derived ethanol as a liquid fuel.
Thus, aside from its traditional use as a beverage or as a source of industrial alcohol, fermentation alcohol is also under study as a source of liquid fuel (i.e., anhydrous ethanol) which can be blended with gasoline in a 1:9 ratio. Since the sugars for the fermentation can be obtained from domestically available surplus grains and cellulosic residues, fermentation alcohol has the potential of significantly decreasing this country's dependence on imported oil.
Recovery of ethanol from the germentation broth is at least a three-step process: (i) distillation of dilute aqueous alcohol to its azeotrope (95.57 percent ethanol by weight), (ii) distillation using a third component--either an organic solvent or a strong salt solution to break up the azeotrope and remove the remaining water, and (iii) distillation to separate water from the third component so that it can be recycled. Trace constituents, including pentanol (fusel oil) and methanol, can be removed by additional distillation, but this is not necessary for ethanol to be blended for example with gasoline.
Analysis of the ethanol-water distillation, using the McCabe-Thiele method for analysis of fractionation columns, indicates the energy-sensitive regimes. Energy consumption greatly increases with decreasing ethanol concentration in the feed below 4 percent alcohol, since a disproportionately larger quantity of feed must be vaporized to obtain the same amount of product. Current fermentation technology results in a product containing 5 to 12 percent ethanol, so this energy problem is avoided. Most of the energy consumption occurs in distilling above 85 percent ethanol; With increasing alcohol product concentration (92.2 percent by weight alcohol), the rectifying operating line approaches the equilibrium line. Hence, more theoretical plates are required. To obtain a column with a reasonable number of plates (fewer than 40) for producing ethanol of higher purity, the slope of the rectifying line must be increased. This requires a higher reflux ratio and therefore more energy input.
By using the McCabe-Thiele method and constructing appropriate diagrams and using appropriate mass and energy balances, it can be shown that the energy content of 12 percent alcohol, distilled to 90 percent purity, is 11 times the energy needed for distillation. Above 90 percent, the energy ratio drops precipitously, and as the azeotrope is approached the distillation energy input approaches the ethanol energy output. Additional energy is required to carry out the other distillations to break the azeotrope.
The problem is to produce dehydrated ethanol in an energy-efficient manner, starting from 90 percent or lower concentrations. One solution is to use a non-distillation process. Aqueous ethanol can be dehydrated by preferential adsorption of water on adsorbents that are inexpensive and require relatively little energy for use or regeneration.
It is therefore a primary object of the present invention to provide an energy efficient means of dehydrating aqueous alcohol mixtures.
A further object of the present invention is to provide an efficient means of enriching dilute aqueous alcohol mixtures.
These and other objects of the present invention will become more apparent from the discussion which follows.